The present invention relates to the field of sensors and transducers, and more particularly relates sensors and transducers utilizing optical fibers as sensing elements.
Transducers are devices which provide a signal representing a phenomenon to be sensed. Common transducers include thermistors and thermoresistors which convert temperature to electrical signals and electrical strain gauges which can be attached to an object and which convert deformation of the object into electrical signals. In various applications, it would be desirable to use optical transducers which convert the phenomenon to be sensed, such as temperature, mechanical strain or pressure, into an optical signal such as a change in the properties of light passing along an optical fiber.
Optical signals can be communicated over considerable distances through fiber optics. It would be desirable to provide sensors which can be conveniently integrated with fiber optic systems. Because fiber optics are of small diameter, an optical sensor which can be integrated with a fiber can be placed readily in an environment having restricted accessibility such as within the interior of a machine or structure. For example, an optical strain gauge sensitive to deformation can be embedded in an object such as a structural member and used to monitor deformation of the object.
There has been a particular need for optical sensing assemblies which can monitor phenomena occurring at several locations. For example, it is often desirable to monitor the deformation of a structural member at numerous locations within the structural member, or to monitor temperature at various points in a room or in an industrial system. It would be desirable to provide monitoring at numerous points without the need for a separate communications channel extending to each point.
Various approaches have been proposed for making optical sensors. Wong et al., Truly Form Birefringent Fibres, Integrated Photonics Research, Post Deadline Papers, 1992, pp. 52-55; Kecuchi et al., Polarimetric Strain and Pressure Sensors Using Temperature-Independent Polarization Maintaining Optical Fibre, Proc. 2nd International Conference Ofs., pp. 395-398 (1984); Dakin et al., Compensated Polarimetric Sensor Using Polarization Maintaining Fibre In A Differential Configuration, Electronics Letters, Vol. 20, No. 1, pp. 51-53 (1984); and Lefevre et al., Optical Fiber Hydrophone and Antenna Associating A Series Of Hydrophones, U.S. Pat. No. 4,882,716, all discuss optical fiber sensors using a special form of optical fiber known as a polarization maintaining fiber as a sensitive element.
A polarization maintaining optical fiber has different propagation constants for light having different polarizations. The term "propagation constant" refers to a measure of the speed with which a light propagates along a path such as a fiber. The propagation constant is also a measure of the wavelength of the light propagating along the path. Propagation constant is commonly denoted by the symbol .beta.. A typical optical fiber includes a core of glass and a cladding formed from glass having different optical properties than the core. The propagation constant of light passing through an optical fiber depends upon the diameter of the core, the indices of refraction of the core and cladding and the wavelength of the light. In one type of polarization maintaining fiber, the core is of a non-circular shape such as an elliptical shape, so that the core has different diameters along different axes transverse to the length of the fiber. The fiber thus has a greater propagation constant for light having its electric field direction aligned with one such axis (the "fast axis") and has a lesser propagation constant for light having an electric field direction aligned with the other axis ("the slow axis"). Other polarization maintaining fibers include features which maintain the core under different stress conditions along fast and slow axes, and thus maintain different indices of refraction for light having field directions aligned with the fast and slow axes. Light having its field direction aligned with the fast axis is said to propagate in the fast polarization mode of the fiber, whereas light having its field direction aligned with the slow axis is said to propagate in the slow polarization mode of the fiber.
In optical fiber sensors as disclosed, for example, in the '716 patent, light is launched into a length of polarization maintaining fiber with the polarization of the light at 45.degree. to the slow and fast axes. The light thus includes components along both axes, propagating in both fast and slow modes. The components propagate at different velocities. Consequently, at the downstream end of the fiber, the components differ in phase from one another. When the components at the downstream end of the fiber interfere with one another, the amplitude or power in the resulting light vanes with the phase difference. The phase difference, in turn, depends upon the difference between the propagation constants for the fast axis and the slow axis and upon the length of the fiber. The phase difference changes in response to stress in the fiber and therefore changes in response to an acoustical field impinging on the fiber.
The '716 patent uses an additional polarization maintaining fiber, referred to as a "compensation fiber" which is not subjected to the acoustical field to be sensed. The compensation fiber has its fast and slow axes disposed at 90.degree. to those of the sensing fiber, so that light aligned with the slow axis of the compensation fiber will be aligned with the fast axis of the sensing fiber and vice versa Changes in the phase difference caused by the effect of temperature on the compensation fiber and on the sensing fiber will cancel one another. As also taught in the '716 patent, plural sensing fibers can be connected in parallel to one another. Such an arrangement requires a complex array of fibers and couplers to direct the light along numerous parallel paths and recombine the light from all of the paths into a common return path. This system also depends on the differences between the lengths of the paths to segregate the signals transmitted along different paths. It requires a pulsed light source and equipment for sorting pulses returned through the common return path according to the times at which the pulses are returned.
It would be desirable to provide a simple optical sensing system capable of sensing a plurality of phenomena, without these complexities in the optical components.